The present invention relates to a method of estimating the BMD of a bone by using radiogrammetry.
Today, Bone Mineral Density (BMD) is most often estimated using Dual X-ray Absorptiometry (DEXA) where two images are taken at two different X-ray energies and where BMD is derived from the image resulting from a subtraction of the two images. It is also possible to estimate BMD using Radiographic Absorptiometry (RA), where the X-ray absorption in the bone is used for determining the BMD of the bone. However, the use of that method requires the use of a calibration wedge in order to determine what grey level in the image corresponds to which thickness of e.g. Aluminium. This is required in order to take into account differences in film type, developing methods, radiation doses, and radiation energy used.
The use of this calibration wedge, though, incorporates the disadvantage that forgetting it or using the wrong wedge will render the image data totally useless, where after the patient will have to be irradiated once again.
It is an object of the invention to provide a method of using standard X-ray images for providing BMD data without having to introduce standard wedges or other elements for use in the calculation.
The present invention relates to the use of values normally derived in radiogrammetry. Until now, radiogrammetry has mainly been used for more coarse predictions, such as predictions of fracture risk and the like. These types of predictions merely relate to the determination of whether a derived value is below or above a given threshold value. Exactly how the derived value generally correlates with e.g. BMD has not been in focus and has not been important, as long as the correlation in the vicinity of the threshold value is acceptable.
In radiogrammetry, the usual manner is to start out with an X-ray image of the bone and therefrom determine the overall width, w, of the bone and either the thickness, t, of the cortical shell of the bone, or the inner width, wl, of the bone. Obviously, w=2t+wl.
As seen in xe2x80x9cBone Densitometry and Osteoporosisxe2x80x9d, H. K. Genant, G. Guglielmi and M. Jergas, Springer, 1998, chapter 14, different manners have been used in the estimation of e.g. the fracture risk of a bone:
the combined cortical thickness (CCT): wxe2x88x92wlxe2x88x9dt,
the cortical index: (wxe2x88x92wl)/wxe2x88x9dt/w,
the cortical area: w2xe2x88x92wl2xe2x88x9dt(wxe2x88x92t), and
the relative cortical area: (w2xe2x88x92wl2)/w2xe2x88x9d(t/w)xc3x97(1xe2x88x92t/w).
However, as has also been acknowledged by the prior art, these values have been found to correlate moderately at best with BMD measured using e.g. DEXA.
It is an object of the present invention to provide a novel manner of estimating the BMD of a bonexe2x80x94a manner in which a standard X-ray image may be used and a standard manner of deriving t, w, and/or wl, may be used, but which has been found to correlate better with e.g. a DEXA-BMD measurement of the bone.
Thus, in a first aspect, the invention relates to a method for estimating the Bone Mineral Density (BMD) of a bone, the method comprising:
obtaining two-dimensional image data comprising information relating to the cortical bone of at least a part of the bone, the image data being data obtained by exposing at least the part of the bone to electromagnetic radiation,
determining a thickness, t, of the cortical bone,
estimating the BMD of the bone as:
BMD=const.Axc3x97t,
xe2x80x83where const.A is determined from a calibration based on values of t and corresponding BMD values for one or more bones.
In the present context, we will denote BMD determined using the method of the invention DXR-BMD.
In the present context, const.A may be a constant value used for e.g. a specific bone (typically of a predetermined type and a predetermined part thereof but for all persons in a predetermined group of persons, such as all Caucasian women.
A major reason for the existence of const.A with those properties is that it has been found that a number of bones in the human vertebrate have very similar shapesxe2x80x94except perhaps for a scaling factorxe2x80x94for large groups of persons, such as all Caucasian women. This means that across a predetermined group of persons, no geometric factor needs to be taken into account in order to adapt the measurement across the group.
It is contemplated that a different value for const.A will be suitable for e.g. Asian women or Caucasian men.
Also, it has been found that the density of non-porous bone is surprisingly constant. Or rather, the natural porosity of the dense bone is quite constant. This is also a reason why the same const.A may be used for a whole group of persons. However, it has been found that the natural porosity of bone varies slightly with age. Thus, in order to obtain an even better BMD measure, it may be desired to have const.A depend on a porosity determined from the image data.
On the other hand, there are factors which relate to the individual person and which are quite useful in a determination of BMD. Factors of these types are age, ethnicity and sex. As to ethnicity and sex, different const.A""s may be provided for different ethnicities and sexesxe2x80x94or an expression for const.A may be provided which depends on these values and is thus adapted to take that into account. As to age, entering the age and in that manner taking the variation of bone porosity with age into account will provide this correction without requiring determination of porosity from the image data.
In these situations, the values or expressions for const.A may be derived on the basis of the values of t and corresponding BMD values for one or more bones as well as information relating to one or more of the factorsxe2x80x94where the factors are those relating to the person having one of the one or more bones.
Preferably, const.A will not depend on any other measure derived from the image data.
A scaling factor derives from different distances from an X-ray source to the bone and from the bone to an image forming means, such as a CCD and an X-ray film. Thus, before deriving t, the image data may be scaled. This scaling may be considered a normalisation of the image data.
In order to obtain a suitable reproducibility, it is preferred that the method comprises:
determining a direction at least substantially perpendicular to a longitudinal axis of the bone,
determining the thickness, t, of the cortical bone along the direction.
In one situation, t may be determined along a single line extending in the direction.
If a better reproducibility of the determination is desired, values of t may be determined for a plurality of lines extending in the direction and being positioned at different positions along the longitudinal direction of the bone, and the BMD may be determined on the basis of t values corresponding to the individual lines. In that situation, the BMD may be determined as: const.A times a mean value of the t values corresponding to the individual lines.
In the above, first aspect, the correlation with t has been found sufficient especially when t is small. However, a second order correction of t2/w has been found to make the BMD determination better for larger values of t.
Thus, in a second aspect, the invention relates to a method for estimating the Bone Mineral Density (BMD) of a bone, the method comprising:
obtaining two-dimensional image data comprising information relating to the cortical bone of at least a part of the bone, the image data being data obtained by exposing at least the part of the bone to electromagnetic radiation,
determining a thickness, t, of the cortical bone,
determining an overall width, w, of the bone,
estimating the BMD of the bone as:
BMD=const.Bxc3x97t(1xe2x88x92t/w),
xe2x80x83where const.B is determined from a calibration where pairs of (t,w) have been calibrated to corresponding BMD values for one or more bones.
The above considerations relating to const.A will also apply to const.B.
Also, in order to increase the reproducibility, the method preferably comprises:
determining a direction at least substantially perpendicular to a longitudinal axis of the bone,
determining a thickness, t, of the cortical bone along the direction
determining an overall width, w, of the bone along the direction.
Again, in one situation, t and w may be determined along a single line extending in the direction.
If a better reproducibility of the determination is desired, pairs of (t,w) may be determined for a plurality of lines extending in the direction and being positioned at different positions along the longitudinal direction of the bone, and the BMD may be determined on the basis of pairs of (t,w) corresponding to the individual lines. In that situation, the BMD may be determined on the basis of mean values of the t and w values corresponding to the individual lines.
In any of the aspects of the invention, the reproducibility may be increased by determining a number of t values, and, for the second aspect, w values, for the same bone and at different positions of the bone, whereby, preferably, t values, and, for the second aspect, w values, are determined for each of more than 10, such as more than 20, such as more than 40, such as more than 60, such as more than 80, such as more than 100, such as more than 110 lines per cm. of the bone and within a predetermined longitudinal part of the bone.
It has been found that measurements from a number of bones, such as the metacarpals and radius/ulna, increase the reproducibility of the BMD determination.
Virtually any tubular bone may be used in the aspects of the present invention. Presently, it has been found that suitable bones are selected from the group consisting of: radius, ulna, tibia, fibula, metacarpal, phalanges and femur.
In the present context, the radiation is preferably X-ray radiation, and the image data are either directly obtained using a CCD or is digital data representing a scanned X-ray image obtained using X-ray film.
When the image data is scanned from an X-ray film, this may be from an X-ray film having a resolution of at least 1 pair of lines per centimetre, such as at least 2 pairs of lines per mm, preferably at least 3 pairs of lines per mm, such as at least 4 pairs of lines per mm, preferably at least 5 pairs of lines per mm, such as at least 6 pairs of lines per mm, such as at least 7 pairs of lines per mm, preferably at least 9 pairs of lines per mm, such as at least 10 pairs of lines per mm.
The scanning may be performed at a resolution of at least 10 lines per cm, such as at least 25 lines per cm, preferably at least 100 lines per cm, such as at least 200 lines per cm, such as at least 250 lines per cm.
Some types of tubular bones may be equally well suited for radiogrammetric measurement around their circumferences. This is the case for the metacarpals. However, this is not the case for all tubular bones, such as for the radius and ulnar. These bones are circularly shaped at the sides thereof pointing away from each other. The sides thereof pointing toward each other, however, are wing-shaped and less suitable for a precise determination of t.
Thus, when the bone is a radius, it is preferred that the t value or values is/are determined on a radial side of the radius, and when the bone is an ulnar, it is preferred the t value or values is/are determined on an ulnar side of the ulnar.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.